Phobos is one of the two moons of planet Mars.

Phobos, is the larger and innermost of Mars' two moons. The sister moon of Phobos is Deimos. Phobos is named after Phobos, son of Ares (Mars) from Greek mythology. Phobos orbits closer to a major planet than any other moon in the solar system, less than 6000 km (3728 miles) above the surface of Mars. Phobos is one of the smaller known moons in the solar system. Phobos's systematic designation is Mars I. The adjectival form of the name is Phobian, and in fairly common usage.

Phobos was discovered by American astronomer Asaph Hall on August 18, 1877, at the US Naval Observatory in Washington, D.C., at about 09:14 GMT (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as August 17 16:06 Washington mean time). . Asaph Hall also discovered Deimos, Mars' other moon.

The notebook of the discovery of Phobos by Asaph Hall is as follows

"I repeated the examination in the early part of the night of [August] 11th, and again found nothing, but trying again some hours later I found a faint object on the following side and a little north of the planet. I had barely time to secure an observation of its position when fog from the River stopped the work. This was at half past two o'clock on the night of the 11th. Cloudy weather intervened for several days.

"On 15 August the weather looking more promising, I slept at the Observatory. The sky cleared off with a thunderstorm at 11 o'clock and the search was resumed. The atmosphere however was in a very bad condition and Mars was so blazing and unsteady that nothing could be seen of the object, which we now know was at that time so near the planet as to be invisible.

"On August 16 the object was found again on the following side of the planet, and the observations of that night showed that it was moving with the planet, and if a satellite, was near one of its elongations. Until this time I had said nothing to anyone at the Observatory of my search for a satellite of Mars, but on leaving the observatory after these observations of the 16th, at about three o'clock in the morning, I told my assistant, George Anderson, to whom I had shown the object, that I thought I had discovered a satellite of Mars. I told him also to keep quiet as I did not wish anything said until the matter was beyond doubt. He said nothing, but the thing was too good to keep and I let it out myself. On 17 August between one and two o'clock, while I was reducing my observations, Professor Newcomb came into my room to eat his lunch and I showed him my measures of the faint object near Mars which proved that it was moving with the planet.

"On August 17 while waiting and watching for the outer moon, the inner one was discovered. The observations of the 17th and 18th put beyond doubt the character of these objects and the discovery was publicly announced by Admiral Rodgers."

The names were suggested by Henry Madan (1838-1901), Science Master of Eton, from Book XV of the Iliad, where Ares summons Fear and Fright.

Jonathan Swift's 'prediction' of Phobos.

In part 3 chapter 3 (the "Voyage to Laputa") of Jonathan Swift's famous satire Gulliver's Travels, a fictional work written in 1726, the astronomers of Laputa are described as having discovered two satellites of Mars orbiting at distances of 3 and 5 Martian diameters, and periods of 10 and 21.5 hours, respectively. The actual orbital distances and periods of Phobos and Deimos are 1.4 and 3.5 Martian diameters, and 7.6 and 30.3 hours, respectively. This is regarded as a fascinating coincidence; no telescope in Swift's day would have been even remotely powerful enough to discover these satellites.

Orbital characteristics of Phobos.

Orbits of Phobos and Deimos (to scale), seen from above Mars' north pole.

Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice a day (every 11 h 6 min). It is so close to the surface (in a low-inclination equatorial orbit) that it cannot be seen above the horizon from latitudes greater than 70.4º.

This low orbit means that Phobos will eventually be destroyed: tidal forces are lowering its orbit, currently at the rate of about 1.8 metres per century, and in 30-80 million years it will either impact the surface of Mars or (more likely) break up into a planetary ring. Given Phobos' irregular shape and modeling it as a pile of rubble (specifically a Mohr-Coulomb body), it has been calculated that Phobos is stable with respect to tidal forces, but it is estimated that Phobos will pass the Roche limit for a rubble pile of its description when its orbital radius drops to about 8400 km, and will probably break up soon afterwards . Because of its shape alone, the gravity on Phobos' surface varies by about 210%; the tidal forces raised by Mars more than double this variation (to about 450%) because they compensate for a little more than half of Phobos' gravity at its sub- and anti-Mars poles.

As seen from Phobos, Mars would be 6400 times larger and 2500 times brighter than the full Moon as seen from Earth, taking up a ¼ of the width of a celestial hemisphere.

As seen from Mars' equator, Phobos would be one-third the angular diameter of the full Moon as seen from Earth. Observers at higher Martian latitudes (less than the 70.4º latitude of invisibility) would see a smaller angular diameter because they would be farther away from Phobos. Phobos' apparent size would actually vary by up to 45% as it passed overhead, due to its proximity to Mars' surface. For an equatorial observer, for example, Phobos would be about 0.14º upon rising and swell to 0.20º by the time it reaches the zenith. By comparison, the Sun would have an apparent size of about 0.35º in the Martian sky.

Phobos' phases, in as much as they could be observed from Mars, take 0.3191 days to run their course (Phobos' synodic period), a mere 13 seconds longer than Phobos' sidereal period.

Physical characteristics of Phobos.

A montage of three separate images taken by Viking 1 on October 19, 1978. The large crater (mostly in darkness) on the upper left of the image is Stickney.

Phobos is a dark body that appears to be composed of carbonaceous surface materials. It is similar to the C-type asteroids. Phobos's density is too low to be pure rock, however, and it is known to have significant porosity.

The Soviet spacecraft Phobos 2 reported a faint but steady outgassing from Phobos. Phobos 2 failed before it could determine the nature of the material. Recent images from Mars Global Surveyor indicates that Phobos is covered with a layer of fine-grained regolith at least meters thick.

Phobos is highly nonspherical, with dimensions of 27 × 21.6 × 18.8 km. It is heavily cratered, and the most prominent surface feature is the large crater named Stickney, after the maiden name of Asaph Hall's wife Chloe Angeline Stickney Hall. Like Mimas's crater Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on the surface were probably also caused by the Stickney impact. The grooves are typically less than 30 m deep, 100 - 200 m wide, and up to 20 km in length.

The unique Kaidun meteorite is claimed to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon.

Phobos and Deimos both have much in common with carbonaceous (C-type) asteroids, with very similar spectra, Albedo and density to those seen in C-type asteroids. This has led to speculation that both moons could have been captured into Martian orbit from the main asteroid belt. However, both moons have very circular orbits which lie almost exactly in Mars' equatorial plane, while captured moons would be expected to have eccentric orbits in random inclinations. Some evidence suggests that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal .

"Hollow Phobos" claims.

Around 1958, Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin. Shklovsky based his analysis on estimates of the upper martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light -one calculation yielded a hollow iron sphere 16 km across but less than 6 cm thick.

In a February 1960 letter to the journal Astronautics, however, Siegfried Frederick Singer, then science advisor to President Eisenhower, came out in support of Shklovsky's theory, going as far as stating that "[Phobos'] purpose would probably be to sweep up radiation in Mars' atmosphere, so that Martians could safely operate around their planet". A few years later, in 1963, Raymond H. Wilson Jr., Chief of Applied Mathematics at NASA, allegedly announced to the Institute of Aerospace Sciences that "Phobos might be a colossal base orbiting Mars", and that NASA itself was considering the possibility.

While various theories attempted to explain the acceleration of Phobos, the reality of the secular acceleration itself (corresponding to an altitude loss of about 5 cm per year) was later subjected to doubt, and the problem vanished on its own by 1969. In any case, the measured density of Phobos is inconsistent with a shell.

Similar "hollow Moon" and "Hollow Earth" claims have been made, none of which have withstood examination.

Exploration missions to Phobos including past missions.

Phobos has been photographed close-up by several spacecraft whose primary mission has been to photograph Mars. The first was Mariner 9 in 1971, Viking 1 in 1977, Phobos 2 in 1988, Mars Global Surveyor in 1998 and 2003, and by Mars Express in 2004. The Phobos 1 probe was lost en route to Mars.

Future missions to Phobos.

The Russian Space Agency is planning on launching a joint sample return mission with China in 2009, called Phobos-Grunt.